Contemporary lenses are designed/optimized to focus on a planar image surface. However, optical lens systems do not generally have their best focus on a planar imaging surface. For example, spherical lens systems tend to best focus on a roughly hemispherical surface, called the Petzval surface. Much of the complexity of lens design is in forcing the lens system to achieve best focus on a planar imaging surface, far away from the Petzval surface.
Developments in sensor technology have yielded somewhat low resolution curved sensors (with the resolution likely to increase in the future) that provide for improved quality of images. However, with such curved sensors, lenses optimized for planar image surfaces are inappropriate.